The present invention relates to the field of particle separation and, more specifically, to recovery of a fine grade coal by means of heavy media suspensions.
As a result of the increasing scarcity of raw materials and fuels, it is desirable to recover as much usable coal as is economically and physically possible.
As is well known, one of the chief difficulties in using coal as a fuel is the pollution that results as a consequence of the high sulfur content in the coal. It has been found, however, that very fine coal is relatively free of sulfur thus rendering the recovery of fine coal fractions highly desirable.
A number of processes have been developed for recovering fine coal on the order of 0.5mm or smaller. One such process is froth flotation, which is, however, less suitable for some types of coal. For instance, oxidized coal cannot efficiently be recovered by froth flotation. A more suitable process is then a separation according to specific gravity using liquid separating media.
It has been proposed to remove the fine particles from a raw coal feed product containing also coarser particles in a classifying step, usually called de-sliming, and separating these fine particles into a fine light fraction and a fine heavy fraction in a separate separating system. To obtain a high efficiency, the classifying step is usually carried out by screening while rinsing the product being screened with water. This involves the introduction of an amount of water which has a diluting effect on the separating medium that has to be compensated for by increasing the specific gravity of the heavy medium with which the product is mixed prior to the specific gravity separation.
According to U.S. Pat. No. 2,932,395 of Apr. 20, 1960, this drawback can be eliminated by rinsing the product being screened with heavy medium in lieu of water. However, as a practical matter, in this process, the de-sliming has to be carried out with a large amount of heavy medium to avoid unsatisfactory removal of the fine particles and accumulation thereof in the heavy medium circulating through the process. As a consequence, the fine particles fed to the separate separating system, which consists of one or more heavy medium cyclones, are accompanied by a comparatively large amount of heavy medium and the solids concentration in the feed to the hydrocyclone(s) is lower than admissible for an efficient separation. As a result, the amount of heavy medium to be treated is substantially increased and the capital investment for the equipment in the medium cleaning system is increased correspondingly. Also, the screening equipment required for the classifying step adds substantially to the capital investment of the plant.
Apart from the initial capital investment in constructing a separating process plant, one of the major cost factors in running such plants resides in the consumption of fresh water needed to efficiently carry out the process and the treatment of the amount of water circulating in the process. Accordingly, the prior art has endeavored to reduce the consumption of fresh water required to successfully effect coal washing and particle separation. However, such processes have, in general, resulted in an increase in equipment cost per unit quantity of coal processed per unit of time as a consequence of the necessity of clarifying or purifying the water used in the process.
Further disclosures representative of the prior art in this field are U.S. Pat. Nos. 2,387,866 of Oct. 30, 1945; 2,623,637 of Dec. 30, 1952; 2,984,355 of May 16, 1961 and 2,998,882 of Sept. 5, 1961.
It is an object of the present invention to provide a coal cleaning and recovery process which effects a substantial saving in capital investment and also in the quantity of water required in the process. Further, it is an object of the present invention to provide a separating process capable of recovering extremely fine particles of coal as well as for providing a process that has an inherent flexibility in selecting the level of discrimination among the particles of coal and refuse to be recovered.
To summarize the process of the present invention, raw coal containing coal and shale in a range of particle sizes is mixed with magnetic particles in suspension and fed to a first stage separator which is preferably a heavy medium cyclone where the raw coal is separated into a heavy fraction and a light fraction. The two fractions are then fed to draining screens and washed on washing screens by spraying with a liquid such as clarified water and fresh water to remove the fine suspension. From the draining screens is derived a first mixture, a portion of which is fed to a second stage separator which is preferably also a heavy medium cyclone. The dilute suspensions drained from the two washing screens are fed separately to mixing tanks which receive separately the heavy and light fractions recovered from the second stage separator. The coarse fractions from the screens are, of course, removed from the process, as coarse clean coal and refuse.
The diluted fractions obtained from the second stage separator are then separately fed to sieve bends to obtain tertiary, relatively coarse light and heavy and relatively fine light and heavy fractions which are then fed to magnetic separators to recover the magnetic particles for re-use in the process. The diluted fine light fraction from which the magnetic particles have been recovered is fed to a thickening cyclone and at least a portion of the liquid recovered from this cyclone is employed as a dilution liquid for the secondary heavy fractions from the second stage separator. With this arrangement, the necessity of using large quantities of either clarified or fresh water in the recovery of the magnetic particles at this stage of the process can be entirely eliminated without any significant reduction in the proportion of magnetic particles recovered.